Shift util for Cats IO

gistfile1.txt

// from https://gitter.im/typelevel/cats-effect?at=5cdae78a5a887e1cd9e8a334

trait Shifter[F[_]] { 
  def shiftFair[A](fa: F[A]): F[A] 
} 

def fairShifter(n: Int): F[Shifter[F]] = 
  Ref.of(0).map { 
    new Shifter[F] { 
      def shiftFair(fa: F[A]): F[A] = ref.modify(i => (i+1, i%n == 0).ifM(shift >> fa, fa) 
    }
  }